1. Field of the Invention
The present invention relates to golf balls maintaining rebound performance and also providing improved resistance to abrasion and improved resistance to wear in an excellent balance.
2. Description of the Background Art
Conventionally, a golf ball having a thread-wound layer formed at a liquid center and a balata cover covering thereof has widely been used by advanced golfers and professional golfers as being excellent in shot feel and controllability. Such a golf ball, however, has a structure that makes the manufacturing process complicated, and is poor in durability and resistance against abrasion.
On the other hand, ionomer resin is excellent in rebound performance, durability, resistance to wear and workability and it is widely used as a covering material. Ionomer resin, however, is stiff and hard and provides uncomfortable feel, impaired spin performance, and poor controllability. Accordingly, covering materials have been developed to provide golf balls with improved, required characteristics which are well balanced.
Japanese Patent Laying-Open No. 62-64378 proposes a cover composition for a golf ball where a fine fiber having an amide group is blended in a cover material containing trans-polyisoprene as a main component to improve rebound property, spin property and cut resistance.
Japanese Patent Laying-Open No. 63-9461 proposes a cover composition for a golf ball where 5-42 parts by mass of trans-polybutadiene, 1-15 parts by mass of fine fiber having an amide group, and 5-30 parts by mass of natural rubber are blended in 100 parts by mass of a cover material containing trans-polyisoprene as a main component, as a cover material that realizes the best possible performance, that is, a flight distance of a multi-dimple type golf ball.
Japanese Patent Laying-Open No. 1-223980 proposes a thread-wound golf ball using a cover material where inorganic single crystal fibers are blended in a trans-1,4-polyisoprene-based cover material to improve a flight distance of the ball.
U.S. Pat. No. 5,716,293 discloses the use of a solid rubber center containing an oily material and a soft cover material to improve a shot feel and to increase a spin amount with a short iron as well. Since this technique uses an oil proof rubber or an ionomer resin having a high hardness for the outer side of the solid rubber center, the rebound performance and the shot feel is yet to be improved.
U.S. Pat. No. 5,935,021 proposes a technique to improve rebound property, durability and cut resistance by blending fibrous aluminum borate whiskers in a main material of a thermoplastic resin or a thermoplastic elastomer. This technique, however, reduces the rebound performance of the cover material by blending the aforementioned whiskers.
Japanese Patent Laying-Open No. 10-179802 proposes a golf ball where a base resin of a cover is formed of a two-component heated mixture, as a main component, of an ionomer resin and a styrene-butadiene-styrene block copolymer having a polybutadiene block containing an epoxy group or a styrene-isoprene-styrene block copolymer having polyisoprene block containing an epoxy group, characterized in that the composition forming the cover has a flexural modulus of 50-300 MPa and a Shore D hardness of 40-60. This technique is intended to improve the shot feel, the spin performance and the flight performance, but the cut resistance is still to be improved.
Japanese Patent Laying-Open No. 10-225532 proposes a cover composition for a golf ball formed of a resin composition in which aluminum borate whiskers are blended in a resin material made of an ionomer resin as a base material to improve durability.
Japanese Patent Laying-Open No. 6-319832 proposes a composition as a cover material formed by blending a core shell polymer made of (a) a core of a rubber-like polymer having an epoxy group or a carboxyl group or an acid anhydride group on the surface and (b) a shell of a glassy polymer, into an ethylene-unsaturated carboxylc acid copolymer, to improve a soft feeling and durability as well as rebound performance.
Japanese Patent Laying-open No. 2001-70478 proposes for a covering material a rubber compound formed of polybutadiene having a cis 1,4 structure content of at least 40% with 5 to 40% by weight of α,β-ethylene unsaturated carboxylic acid, 5 to 40 weight of a metal oxide, and 0.1 to 5 parts by weight of a polymerization initiator blended therewith. However, it is not satisfactory in resistance to wear, resistance to abrasion, and rebound performance.
The Journal of the Society of Rheology, Japan, Vol. 25 (1997) has reported a development of a micro disperse system super-fine fiber reinforced composite prepared from a rubber-polyolefin-nylon ternary graft copolymer as an application of the plastic field.
Note that U.S. Pat. No. 5,697,856 discloses that a golf ball has a core formed using polybutadiene rubber as a rubber base material and containing a cis 1,4 structure content of no less than 90% prior to vulcanization and a trans structure content of 10 to 30% after vulcanization and the core also has a hardness decreasing in difference in the direction of the depth.
These conventional techniques cannot improve the rebound performance, the spin performance, the cut resistance, durability, and feeling on the whole.